ARTEMIS-6085 Fix NPE from race in MQTTStateManager.getSessionState#6480
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getSessionState() used a non-atomic containsKey()/get() pair on a ConcurrentHashMap. During link stealing (two connections sharing a client ID) removeSessionState() can run between the two calls, so get() returns null and handleLinkStealing() dereferences it via .getSession(), throwing a NullPointerException. Use computeIfAbsent() for an atomic get-or-create that never returns null. Adds a concurrency regression test that reproduces the NPE.
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@k-krawczyk many thanks for the great contribution! |
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What
Fixes a
NullPointerExceptionthrown fromMQTTProtocolHandler#handleLinkStealing()(getSessionState(clientID).getSession()).Root cause
MQTTStateManager.getSessionState(String)used a non-atomiccontainsKey()+get()on aConcurrentHashMap. During link stealing (two connections sharing a client ID),removeSessionState()can run on another thread between the two calls, soget()returnsnulland the caller dereferences it with.getSession().The race is still present on
main.Fix
Replace the
containsKey()/get()/put()block with an atomiccomputeIfAbsent()— a get-or-create that can never returnnull. Behaviour is otherwise unchanged.Test
Adds
MQTTStateManagerTest, a concurrency regression test that runsgetSessionState()against concurrentremoveSessionState()on the same client ID and asserts a non-null result. It fails (NPE/null) on the pre-fix code in ~1s and passes after the fix. The fullartemis-mqtt-protocolmodule suite is green (12/12).JIRA: https://issues.apache.org/jira/browse/ARTEMIS-6085